This invention relates to the field of display assemblies and more particularly to backlighted liquid crystal display assemblies.
Liquid crystal displays (LCD's) are well known for their ability to form predetermined images of alpha-numeric characters and other patterns in response to appropriate electrical signals. Because LCD's are passive displays, i.e., the images themselves do not emit light, an external light source is necessary to illuminate the display. Under normal room lighting conditions, ambient light provides sufficient external light intensity to view the display, however, a supplemental light source is necessary when ambient light intensity is low.
Supplemental light sources are commonly positioned behind the LCD to "backlight" the display. In backlighting, light from the supplemental light source is transmitted through the background or non-image area of the display while the formed images obstruct light transmission, resulting in dark images against a bright background.
An exploded perspective view of a well known prior art LCD assembly with supplemental backlighting is illustrated in FIG. 1. The assembly utilizes a "light guide" to provide backlighting for LCD 10. Images 12, for example the illustrated numbers "1","2" and "3", are formed against a background area 14 in response to appropriate electrical signals. The light guide 16 is typically made from a transparent plastic slab having an upper surface 18, a lower surface 20 (not visible in FIG. 1), and four bordering surfaces 22, 24, 26, and 28 (only surfaces 22 and 24 are visible in FIG. 1). Light sources 30 and 32, typically incandescent lamps or light emitting diodes (LED's), are respectively positioned in notches 34 and 36. Bottom surface 20 has two planes 38 and 40 depressed into the bottom surface, thereby forming a V-shaped wedge. A reflective coating covers bottom surface 20, depressed planes 38 and 40, areas 42 and 44 of top surface 18, and bordering surfaces 22, 24, 26, and 28, with the exception of notches 34 and 36 which remain transparent.
In operation, light emitted from sources 30 and 32 travels the length of the light guide toward planes 38 and 40, strikes the reflective coating thereon, and is reflected up through transparent surface 18. The light then passes through background area 14 of LCD 10, but is obstructed by images 12. Thus, images 12 appear dark against bright background area 14.
An important criterion for any supplemental light source is that it provide an even distribution of light intensity across the surface of the display, i.e., the light intensity at any lighted point on the display should be substantially the same as it is at any other lighted point. When this condition is not meet, the supplemental light source is said to have "hot spots", i.e., areas wherein the light intensity is brighter than other areas. Although the light guide probably provides the best distribution of light intensity of any supplemental light source, the distribution is not perfectly even.
To complicate the problem, the demand for smaller and more compact electronic products has reduced the space available for supplemental light sources. Consequently, when a light guide is used for supplemental lighting the thickness (distance between surfaces 18 and 20) of the light guide must be correspondingly reduced. As the light guide is made thinner, the distribution of light intensity becomes more irregular, resulting in objectionable hot spots.